Recently, as exhaustion of existing energy resources such as petroleum and coal is estimated, interests on alternative energies for substituting them are more increased. Among the alternative energies, a solar cell is particularly spotlighted since its energy resource is abundant and it does not cause any environmental pollution. A solar cell is classified into a solar heat cell that generates a vapor required for rotating a turbine using a solar heat, and a solar light cell that converts photons into electric energy using the properties of a semiconductor. Generally, a solar cell calls the solar light cell (hereinafter, the term ‘solar cell’ is used).
Referring to FIG. 1 showing a basic structure of a solar cell, the solar cell has a junction structure of a p-type semiconductor 101 and a n-type semiconductor 102 like a diode. If photons are input to the solar cell, the photons are reacted with materials of the semiconductor to generate electrons of (−) charge and holes of (+) charge caused by removal of the electrons, thereby allowing flow of electric current as they are moving. It is called photovoltaic effect, and the solar cell gives power using this photovoltaic effect.
Such output features of a solar cell are generally evaluated by measuring an output current-voltage curve using a solar simulator. On the curve, a point at which a multiplication Ip×Vp of an output current Ip and an output voltage Vp is maximum is called a maximum output Pm. Also, a value obtained by dividing Pm by a total optical energy (SI: S is an area of an element, I is an intensity of light irradiated to the solar cell) input to the solar cell is defined as conversion efficiency η. In order to increase the conversion efficiency η, it is required to increase short-circuit current Isc (an output current at V=0 on the current-voltage curve) or open voltage Voc (an output voltage at I=0 on the current-voltage curve), or to increase FF (fill factor) indicating how the output current-voltage curve is close to a square shape. As FF has a value closer to 1, the output current-voltage curve approximates to an ideal square shape, and it means that the conversion efficiency η is increased.
Among three factors determining the efficiency of a solar cell, the short-circuit current Isc is determined based on absorptance or reflectance of the solar cell with respect to a light irradiated to the solar cell. Thus, there are many efforts to decrease reflectance, and representatively, there is used a method for forming an anti-reflection film on a front surface of a solar cell to which photons are irradiated or a method for minimizing an area of a front electrode. In addition, there is used a method for reducing reflectance by etching a front surface of a silicon wafer to form unevenness thereon, which is called ‘texturing’.
However, a polycrystalline silicon wafer shows irregular surface texturing as shown in FIG. 2 due to different etching rates depending on crystal surfaces, so the reflectance is not so greatly reduced using the texturing process. In this reason, there have been made many endeavors to solve the above problem, and the present invention is made based on the above.